The present invention relates to a driving unit for driving a transducer that vibrates an elongated vibrating body. More specifically, the present invention pertains to a driving unit that is suitable for an apparatus that levitates an object by acoustic radiation pressure of sound waves.
Japanese Laid-Open Patent Publications No. 7-137824 and No. 9-202425 each discloses an apparatus that has such driving unit. Each apparatus includes an elongated vibrating body, a vibrating transducer, and a receiving transducer. The vibrating transducer is located on one end of the vibrating body and generates oscillating wave (sound wave). The receiving transducer is located on the other end of the vibrating body and receives the oscillating wave. The vibrating transducer in cooperation with the receiving transducer generates traveling waves on the vibrating body, which levitates an object from the surface of the vibrating body and transports the levitated object by acoustic radiation pressure of the traveling waves. The receiving transducer has a load circuit to generate traveling waves on the vibrating body. The load circuit converts vibrational energy of the transducer to electrical energy. The load circuit has a resistance that consumes the electrical energy in the form of heat. The above publication No. 9-202425 discloses an apparatus for transporting a plate-like wide object. The apparatus generates traveling waves on several vibrating bodies arranged parallel to one another to transport a wide object.
The apparatus uses a Langevin transducer that uses a piezoelectric element as a vibrating transducer. The transducer that uses a piezoelectric element is excited at a resonance frequency of the vibrating system, which vibrates the vibrating body at a required magnitude (amplitude). The vibrating system means all parts of the apparatus which can be vibrated.
The above publications discloses that the vibrating transducer is excited at a resonance frequency of the vibration system but does not disclose any controlling method of an oscillator that excites the transducer.
A typical driving system for stably driving a piezoelectric transducer includes a bridge feedback oscillator or a vibration feedback oscillator (see The Ultrasound Handbook). Assume that the terminal voltage of an electrostriction transducer (piezoelectric transducer) is represented by V, the terminal current is represented by I, the damping admittance is represented by Yd, and the dynamic admittance is represented by Ym. The bridge feedback oscillator utilizes the characteristic that the current YmV that flows through a piezoelectric element with the dynamic admittance is proportional to the vibration speed. More specifically, a feedback oscillation is performed by inserting a bridge circuit between a power amplifier and a transducer and picking up the voltage, which is proportional to the vibration speed. FIG. 5 shows a bridge circuit, which has an electrostriction transducer 40 on one side. The damping admittance Yd of the bridge circuit is balanced such that the bridge output voltage is zero when the electrostriction transducer 40 is damped. Thus, the bridge output, which is proportional to the vibrating speed, is obtained when the electrostriction transducer 40 is vibrated.
In the vibration feedback oscillator, an electrode of the vibrating transducer, more specifically, piezoelectric transducer, is divided and part of the electrode is used as a pickup for detecting the output, which is proportional to the vibration speed. Based on the output voltage, the vibrating transducer is oscillated.
However, in the bridge feedback oscillator, it takes a lot of trouble to adjust a constant number of the circuit at a bridge portion. Further, the constant number needs to be adjusted every time the load changes.
On the other hand, in the vibrating feedback oscillator, a pickup is mechanically coupled to a piezoelectric transducer. Thus, the reliability or the mounting space can be concerned.
When the vibrating transducer and the receiving transducer are apart from each other such as in a case when an elongated plate-like vibrating body is used, although the vibrating body is vibrated in a suitable manner on the side close to the vibrating transducer, the side of the vibrating body close to the receiving transducer might not be vibrated in a suitable manner. In the above described prior art oscillators, only the components corresponding to the vibration of the vibrating transducer are fed back to control a vibration state of the vibrating body. Thus, even when such feedback is performed, the entire vibrating body might not vibrate evenly.
Also, an elongated vibrating body has several points of resonance frequencies. Therefore, if only the side of the vibrating body close to the vibrating transducer is observed, the vibrating body might be vibrated in an undesired mode. Particularly, when generating standing waves on the vibrating body, the vibrating body is preferably vibrated at a resonance point. However, when generating traveling waves on the vibrating body, the vibrating body is preferably vibrated at a frequency that is not the resonance point. In this case, the vibrating body is preferably vibrated at a frequency in the vicinity of the resonance point and by a predetermined phase difference from the phase of the receiving transducer. Thus, although the electrical impedance of signals from the vibrating transducer is detected, the frequency that is suitable for traveling waves is not obtained.